The present invention relates generally to lasers and, more particularly, to a laser resonator which corrects distortion in the wavefronts and frequency walkoff.
The optical elements of the laser resonator determine the spatial coherency of the laser beam which directly affects propagation and focusing capabilities. Some of the problems associated with the laser beam formation are vibration of reflecting surfaces, misalignment, aberrations in the lasing medium, index inhomogeneties, etc.
Prior art solutions to these problems require a high degree of accuracy in the optical components used (typically fabrication accuracy to .lambda./10 or better), and mechanically stable oscillator cavities with low Fresnel numbers (typically a.sup.2 .lambda.L.about.1). A mode-selecting aperture is conventionally used to select the lowest order transverse mode when optimum spatial coherence and beam propagation are desired. Accurate alignment of the focusing elements, such as the cavity mirrors, aperture, and the like is critical in the conventional laser resonator. A large mode diameter is generally desirable to achieve efficient extraction of laser energy using conventional plane or curved mirror laser resonators. This can be achieved only at the expense of even more stringent optical quality, alignment and lasing medium uniformity.
Another approach for producing a large mode diameter while providing better performance involves the use of a spatial filter. This requires placing two lenses and a pinhole aperture within the laser cavity in the beam path. However, the disadvantages of this approach include additional elements which must be aligned, the same great sensitivity to optical aberrations of the medium or the optical elements, and the resultant loss of power upon the aperture. An additional difficulty is that high power operation is precluded by laserinduced breakdown at the aperture due to the presence of a tightly focused beam and high power density.
Prior attempts to correct unavoidable aberrations of the medium or optics have utilized a correction device external to the laser cavity. Two examples are the mechanically deformable mirror described in U.S. Pat. No. 3,731,103, and the technique of U.S. Pat. No. 4,005,935.
More recently several attempts have succeeded in correcting phase front distortions in a laser cavity by using the mechanically deformable mirror inside a laser cavity. This technique is described in "Experimental Studies of Adaptive Laser Resonator Techniques", R. R. Stevens and R. C. Lind, with anticipated publication in Optics Letters, and "Adaptive Laser Resonator", R. H. Freeman et al, Opt. Lett., Vol. 2, No. 3, March 1978.
Drawbacks of this type of system include slow response times, need for external beam sampling to provide a feedback loop for the mechanical mirror servo system, and general system complexity resulting in high system cost and lower reliability.
One solution to the above problem is the use of a nonlinear phase conjugation device called a stimulated Brillouin scattering (SBS) device within the laser resonator cavity. The SBS device corrects for distortions in the wavefronts of the laser beam by reflecting the complex phase conjugate image of the distorted incident optical wavefront. When the reflected wave encounters the abnormality which initially caused the distortion, because it is the phase conjugate image of the distorted wave, it interacts with the abnormalities to form a plane wave. But each reflection off of the SBS device shifts the wavelength at the reflected signal as a consequence of the moving grating. This frequency shift is typically hundreds of megahertz and accumulates progressively with each reflection off of the SBS device. As a consequence, within a few iterations the frequency of the reflected wave "walks away" from the gain region. Bandwidth of the laser medium and efficient energy extraction comes to a halt. Thus, a long-pulse mode requiring many successive reflections becomes very difficult to obtain, if not impossible.
These drawbacks have motivated a search for an improved laser resonator having an SBS device therein.